Level changing direct coupled amplifier



Otl1, 1960 B. KLESTADT 2,956,236

LEVEL CHANGING DIRECT COUPLED AMPLIFIER Original Filed May 10, 1955United States Patent O LEVEL CHANGING DIRECT COUPLED AMPLIFIERContinuation of application Ser. No. 507,695, May 10, 1955. ThisVapplication July 29, 1959, Ser. No.

4 Claims. (Cl. S30-123) This invention relates to direct coupledampliiiers, and particularly to circuits for changing the referencevoltage level about which a signal varies.

This application is a continuation of applicants prior copendingapplication Serial No' 507,695, filed May i 10, 1955, now abandoned.

When interconnecting electronic equipment, it is sometimes found thatthe signals developed by one electronic device may not be directlyapplied to another device, due to an incompatibility in referencevoltage levels. Some signals appear at a single terminal with respect tozero voltage or ground, while other signals appear at two terminalswhich are balanced With respect to ground. Occasionally, the voltagereference level .is a positive or negative voltage with respect toground, rather than zero voltage.

As a specific example, it may be desired to determine the ditferencebetween two signals to be applied to test equipment designed toaccommodate signals varying about a predetermined reference voltagelevel, such as ground, for example. To use the test equipment, itV maybe necessary to change the operating or reference level of the inputsignals from a positive or negative voltage to zero voltage or ground.It is desirable that the level changing be accomplished Withoutdisturbing the source of the signals, particularly where the source is ahigh impedance circuit.

It is, therefore, an object of the present invention to provide acircuit for changing the voltage reference level of applied inputsignals.

Another object of this invention is the provision of a circuit forchanging a signal appearing at a single terminal with respect to a xedpotential to a signal appearing at two terminals which is balanced Withrespect to a fixed potential.

Yet another object of the invention is -to provide a circuit forchanging a signal appearing at two terminals balanced with respect to afixed potential to a signal appearing at a single terminal with respectto a fixed potential.

Still another object of the invention is the provision of a circuit forcombining two signals into a single signal varying about a predeterminedvoltage reference level.

A further object of the present invention is to provide a level changingcircuit whose output voltage reference level is relatively insensitiveto arbitrary variations of input voltage reference level.

An even further object of the invention is the provision of a levelchanging circuit having a high input impedance.

A still further object of the invention is to provide a level changingcircuit which accommodates signals of an extremely low frequency as Wellas higher frequency signals.

A level changing circuit constructed in accordance with this inventionmay utilize a differential amplier responsive to input signals. Theinput signals may vary about relatively high values of voltage, and itmay be desired to shift the average voltage value of the signals toground or some other reference potential. Output signals from thedilferential amplifier are provided to an impedance network including airst pair of series impedance elements and a second pair of seriesimpedance elements in parallel with the iirst pair of elements. Outputsignals may be taken from the junction points of the irst and secondpairs of series elements. The input circuit of a D.C. (direct-coupled)ampliier is coupled to the midpoint of the second pair of seriesimpedance elements andl is also coupled to a source of a predeterminedreference voltage. The D.C. ampliiier provides a degenerative feedbacksignal, coupled through a cathode follower, to the midpoint of the iirstpair of series impedance elements. The feedback loop, and the signalthus provided, compensate for changes in the reference levels of theinput signals.

The novel features of the invention, as well as the invention itself,may best be understood when considered in the light of the followingdescription, including the accompanying drawing, the sole gure of whichrepresents an arrangement for practicing the invention.

Referring to the drawing, a level changing circuit constructed inaccordance with the invention may derive signals from a pair of signalinput terminals, here designated signal input terminal 10 and signalinput terminal 12. Signal A from input terminal 10 is applied to a iirsttriode 18 and signal B from input terminal 12 is applied to a secondtriode 30. Signal A from input terminal 1t) is applied to the grid 22o-f the iirst triode 18 and a grid resistor 14 connected to ground. Theplate 20 of the rst triode 18 is coupled through a plate load resistor26 to a B+ supply 28. Similarly, signal B is applied to the grid 34 ofthe second triode 3i) and to a grid resistor 16 connected to ground. Theplate 32 of the second triode 30 is coupled through a plate loadresistor 38 to the B+ supply 28. The cathodes 24, 36 of the iirst andsecond triodes 18, 30 are connected together and both are also coupledthrough a common cathode resistor 40 to a B- supply 42.

The first and second triodes 18, 3G together -form a differentialamplifier. The resistor 40 in the common cathode connection of thetriodes 18, 30 may be replaced, if desired, by a constant currentdevice.

The plates 20, 32 of the triodes 18, 30 are connected to an impedancenetwork. The plate 20 of the rst triode 18 is connected to a resistor44. The plate 32 of the second triode 30 is connected to a resistor 46.The resistors 44 and 46 are connected together between their free endsby a first pair of series impedance elements and a second pair of seriesimpedance elements in parallel With the first pair of elements. The rstpair of series impedance elements comprises a pair of series-connectedresistors 48 and 50 having matched values of resistance and being joinedat junction 49. The second pair of series impedance elements comprisesanother series-connected pair of resistors 52, 54, also having matchedvalues of resistances and being joined at junction 53. Output terminals68, 70 for .the circuit are connected to the junction of resistors 44,46 with the two pairs of series elements.

The input terminal of a D.C. ampliiier 56 is connected to the junction53 of the second pair of series elements. A circuit which may beemployed for the D.C. amplifier 56 is shown and described as Fig. 5.33at page 195 of the book Electronic Analog Computers, by Korn and Korn,published by McGraw-Hill Book Company, Inc. (1952). The D.C. ampliiier56 is also coupled to. a reference voltage source 58 which may be anydevice t0;

Patented ocr. i1, issu` providing a predetermined reference voltage. Thereference voltage source 58 may be, for example, a D.C. supply, a commonground connection, or a variable source of potential. The outputterminal of the D.C. amplifier 56 is coupled to a device having lowoutput impedance, in this case to the grid 62 of a third triode 60connected as a cathode follower. The plate of the third triode l60 isconnected to the B+ supply 28. The cathode 64 is connected -to a cathodeload resistor 66 the `other end of which is connected to the B- supply42. The cathodeV 64 is also connected to the junction 49 of the firstpair of series impedance elements.

In operation, push-pull or single-ended signals at various voltagereference levels may be applied at the signal input terminals 10, 12 andthence to the first and second triodes 18 and 30. The grid resistors 14and 16 establish a limiting impedance between the triode grids 22, 34and ground. The first and second triodes 18 and 30 operate as adifferential amplifier. They therefore isolate the signal sources fromthe remainder of the system, and provide, in effect, a low impedancesource. The signal sources connected to the signal input terminals and12 are thus free of variable loading effects from the present levelchanging circuit. The coupling of the triode cathodes 24, 36 to the B-supply 42 permits the differential amplifier to operate, within limits,with signals provided at a negative level.

In well known fashion, the differential amplifier provides outputpotentials dependent upon the levels of the input signals. These outputpotentials are to be adjusted to vary about the desired reference levelwhich is the potential of the reference voltage source 58. The signalsfrom the differential amplifier are equally attenuated in the resistors44 and 46, which resistors are of equal value. Changing of the level ofthe differential voltage to the desired reference level is accomplishedby the circuitry connected to the other ends of resistors 44 and 46.

A voltage dierence across the second pair of series impedance elementsprovides a voltage at junction 53 which is the average of the outputvoltages. Viewed in a transient condition, before a steady state hasbeen achieved, the action is as follows. The average D.C.

- level, and the desired voltage level from the reference voltage source58, are applied to the D.C. amplifier 56. The D.C. amplifier 56 providesan output signal which is proportional to the difference between the twopotentials. The D.C. amplifier 56 additionally inverts the phase of thesignal, so that if the difference is positive the output signal of theD.C. amplifier 56 is negative. The output signal of the D.C. amplifier56 is used for degenerative feedback and brings the average D.C. outputvoltage toward the level of' the desired reference voltage. The gain ofthe D.C. amplifier 56, and the attenuation provided by the resistors,are the only factors which must be related so as to provide the desiredcompensation effects. With this accomplished, the feedback loop isstable at any instant in time when the output signals vary about thedesired level.

The cathode follower 60 responsive to the D.C. amplifier 56 provides alow output impedance for ease of types of electronic equipment. It willbe apparent that the arrangement may be employed as a phase inverter forsingle-ended signals supplied to either input. If single-ended signalsare used, the output will nonetheless vary around the desired referencepotential.

Thus, there has been described an improved level changing circuit forchanging the reference level about which a signal varies. Signalsderived from a differential amplifier are adjusted by feedback circuitryto vary about a predetermined reference level.

What is claimed is:

l. A circuit for changing a voltage difference between two sources to avoltage variation about a fixed voltage reference level comprising adifferential amplier responsive to the voltage output of each of saidtwo sources, a first impedance network having a first pair of matchedresistors connected serially between the voltage outputs of saidamplifier, a second impedance network having a second pair of matchedresistors connected in parallel with said first impedance network, anadjustable voltage source, means for developing a signal which isproportional to the difference between the potential at the junctionbetween the second pair of matched resistors and the potential of saidadjustable source, said means comprising a direct-coupled amplifierhaving its input coux pled to the junction between the resistors of saidsecond changing the voltage level at junction 49 of the first pair l ofseries impedance element. In accordance with Well known cathode followeraction, lthe potential of the cathode 64 of the third triode 60 followsthe potential of the grid 62, which is the output of the D.C. amplifier56. Thus, the degenerative feedback voltage is applied to the junction49 of the first pair of series impedance elements.

Thus, it may be seen that the D.C. potential about which the outputsignal varies is dependent upon the reference voltage source 53. Theaverage potential of the output signal is in effect measured by thesecond pair of series resistors 52, 54 and compensated for bydegenerative feedback. The average voltage level of the input signals isautomatically altered to a Vdesired level, so that the loutput signalmay be applied directly to common impedance network and its outputcoupled to the junction between the resistors of the first impedancenetwork, said proportional signal being applied to the junction of saidfirst pair of resistors.

2. In combination: a first impedance network having a first pair ofmatched impedance elements connected serially between a pair of signalsources, a second impedance network having a second pair ofseries-connected matched impedance elements connected in parallel withsaid first impedance network, an adjustable voltage source, means fordeveloping a signal which is proportional to the difference between thepotential at the junction between the second pair of matched impedanceelements and the potential of said adjustable source, said meanscomprising a direct-coupled amplier -having 4its input circuit coupledto the junction between the impedance elements of said second impedancenetwork land its output circuit coupled to the junction between theimpedance elements of said first impedance network, said proportionalsignal being applied to the junction of said first pair of impedanceelements.

3. A circuit for changing the reference Voltage of input signalscomprising isolating means responsive to input signals applied betweentwo input signal terminals for developing isolated signals proportionalthereto, first and second pairs of matched series-connected impedanceelements, each pair respectively coupled to said isolating means andhaving said isolated signals applied thereacross, the reference voltageof said isolated signals appearing at the junction between said firstpair of seriesconnected elements, a source of a predetermined referencevoltage, and feedback means coupled to said source and to the junctionbetween said first pair of elements for developing a difference voltagebetween the potential at said junction of said second pair of impedanceelements and said reference potential, said feedback means being coupledto the junction between said second pair of series-connected elements toapply said difference voltage thereto for causing said isolated signalsto vary about said predetermined reference voltage.

4. A circuit for combining two input signals into a single signalvarying about Ia predetermined reference voltage comprising signalcombining means responsive to two input signals for developing a singlesignal, first and second pairs of matched series-connected impedanceelements coupled in parallel to said signal combining means and havingsaid single signal applied thereacross, a source of a predeterminedreference voltage, and difference means coupled to said source and tothe junetion between said first pair of elements for developing adiiference voltage between the potential at said junction of said l'stpail' of impedance elements and said reference Voltage, said differencemeans being coupled to the junction between said second pair ofseries-com nected elements to apply said diierence voltage thereto forcausing said single signal to vary about said predetermined referencevoltage.

References Cited in the le of this patent UNITED STATES PATENTS ChaseNov. 2, 1954 Russell Jan. 8, 1957

